Flat gaskets must, on the one hand, adjust themselves to irregularities in the faces to be sealed (micro-sealing) and, on the other hand, absorb the preliminary tension imparted by the screw forces during continuous duty without further deformation.
To improve micro-sealing the flat gaskets can be provided with a soft top coating, which is made of soft metal or plastic. In order to attain an improved seal, the coating should firstly be soft, so that it adapts well to the irregularities of the mating faces to be sealed in a cold static state, even during installation. Secondly, the sealing effect should also continue to exist during subsequent operation over as long a time of use as possible. In the case of severe dynamically stressed gaskets, such as cylinder head gaskets or exhaust gaskets, which work at high temperatures and under severe pressure loading, there is a tendency for the soft coatings of blowing out, so that adequate sealing is no longer assured. Therefore, flat gaskets attaining a good seal under the said conditions were required.